As is understood, a force or stress applied to a piezoelectric material leads to an electric charge being induced across the material, and conversely, the application of a charge or electric field to the same material results in a change in strain or mechanical deformation. A particular configuration utilizing this property for various uses such as sensing, actuation, and energy harvesting is the piezoelectric cantilever. As a sensor, typically the piezoelectric cantilever features a piezoelectric material integrally attached to a cantilever in a manner such that deflection of the cantilever by some stimulus generates or alters deflection of the cantilever, subsequently generating or altering stress on the piezoelectric material, and generating or altering a voltage produced as a result of the stress. See e.g. U.S. Pat. No. 7,104,134 to Amano et al., filed Mar. 5, 2004 and issued Sep. 12, 2009; see also U.S. Pat. No. 6,453,748 to Pryor, filed Dec. 15, 1999 and issued Sep. 24, 2002, among others. As actuators, the tendency of piezoelectric materials to deform under the influence of an applied voltage is typically exploited to affect movement or manipulation. See e.g., U.S. Pat. No. 5,780,727 to Gimzewski et al., filed Sep. 12, 1994 and issued Jul. 14, 1998, among others. As energy harvesting devices, typically the cantilever is driven to oscillate as a result of external ambient influences, and resulting energy arising from deformation of the piezoelectric is collected through electrodes typically arranged in a d31 or d33 configuration. See e.g., U.S. Pat. No. 7,687,977 to Xu, filed Apr. 10, 2006 and issued Mar. 30, 2010; see also U.S. Pat. No. 7,521,841 to Clingman et al., filed Feb. 1, 2006 and issued Apr. 21, 2009; see also U.S. Pat. No. 7,777,396 to Rastegar et al., filed Jun. 6, 2006 and issued Aug. 17, 2010, among others. Generally approaches concentrate on either direct mechanical influences such as vibration of a connecting member or structure to generate cantilever deflection or provide cantilever oscillation, or concentrate on ambient heat to provide cantilever deformation based on contrasting thermal characteristics within the cantilever. Generally forces arising from thermal creep on a cantilever have not been exploited to provoke cantilever deflection.
Thermal transpiration or thermal creep refers to the thermal force on a gas due to a temperature difference. A well-known device which relies on thermal transpiration is Crookes' Radiometer, also known as a light mill. Generally the light mill is a small chamber containing typically four or more vanes mounted symmetrically around a vertically-oriented axle, with opposing sides of each vane generally parallel to the axle. Light impinging on the chamber generates a force of the vane from hotter to opposing colder sides as air molecules in the vessel strike on the vanes. See e.g., Scandurra et al., “Gas kinetic forces on thin plates in the presence of thermal gradients,” Physical Review E 75(2) (2007), among others. A variety of devices have exploited the resulting rotation of vertical-surface driven light mills. See, for example, U.S. Pat. No. 4,410,805 issued to Berley, issued Oct. 18, 1983, and see U.S. Pat. No. 4,397,150 issued to Paller, issued Aug. 9, 1983, and see U.S. patent application Ser. No. 14/288,253, filed by Nutter et al. on May 27, 2014 and published as US 2015/0013337 A1 on Jan. 15, 2015, and see U.S. Pat. No. 4,926,037 issued to Martin-Lopez, issued May 15, 1990, and see U.S. Pat. No. 9,106,112 B2 issued to Farquharson et al., issued Aug. 11, 2015. Thermal transpiration has also been employed to address challenges inherent to miniaturized moving parts, such as micropumps. See e.g. U.S. Pat. No. 5,871,336 issued to Young, issued Feb. 16, 1999, and see U.S. Pat. No. 8,235,675 issued to Gianchandani et al., issued Aug. 7, 2012, and see U.S. Pat. No. 5,611,208 issued to Hemmerich et al., issued Mar. 18, 1997, among others. In these applications, asymmetric momentum transfer between gas molecules and channel walls results in an effective momentum transfer to the channel walls in the direction opposite to a temperature gradient, generating a force parallel to the channel surface.
Disclosed here is a piezoelectric cantilever which generates thermal creep and corresponding momentum transfers in order to generate cantilever deflection and stress on a coupled piezoelectric material in response to a radiant flux such as light. The cantilever comprises a vane having planar surfaces of differing emissivities, with a piezoelectric material coupled to a section of the vane. When a radiant flux such as light simultaneously impinges the adjacent high emissivity and low emissivity surfaces, the differing emissivities generate a thermal creep force and deflection of the vane, with subsequent stress on the coupled piezoelectric material. Electrical power is harvested via electrodes in electrical contact with the piezoelectric material.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.